Method for preparing wetproofed catalyst particles and particles produced thereby

ABSTRACT

Carbon particles non-uniformly and non-continuously coated with polytetrafluoroethylene (PTFE) are produced by adding a PTFE emulsion to the carbon particles at a rate of less than approximately 100 ml/100 grams. The result is a mixture of PTFE coated carbon particles, some having a higher weight percent PTFE resin thereon and others having a lower weight percent PTFE resin thereon. Such a mixture of non-uniformly coated particles gives improved reaction efficiency when used as a solid catalyst in a packed bed tower reactor through which fluid reactants are flowed.

BACKGROUND OF THE INVENTION

This invention relates to a method for preparing wetproofed catalystparticles and, more particularly, to a method for non-uniformlydepositing polytetrafluoroethylene resin on carbon particles.

Recently it has been proposed to use wetproofed catalyst particles topromote a number of different redox reactions between two fluid phases,at least one of which is an aqueous liquid. An advantage to usingwetproofed catalyst particles to promote such reactions is that thewetproofed catalyst particles are not "flooded" by the aqueous liquidphase. This is because coating of hydrophobic material on the catalystsurfaces is accomplished in a discontinuous manner.

Take, for example, the oxidation of liquid sodium sulfide to sodiumpolysulfide. When introduced, the liquid sodium sulfide reactant willcompletely surround all of the surfaces of the catalyst but it will notwet the treated portions (those coated with the hydrophobic material).That is, the contact angle between the liquid and the hydrophobicmaterial is so high and the surface adhesion so slight that when theoxidant gas is introduced, it will readily displace the liquid in thewetproofed areas of the catalyst. The adjacent uncoated areas of thecatalyst surface will, on the other hand, be wet by the liquid sodiumsulfide. This, then, forms the requisite locus of interfacial contactbetween the gaseous oxidant, liquid sodium sulfide reductant, andcatalyst surface. It eliminates the need for the gas to diffuse throughthe liquid which would otherwise "flood" the surface of the catalyst andthus increases the rate of reaction.

This advantage and others are all discussed in detail in the Smith andSanders patents, assigned to the assignee of the present invention.Illustrative are Canadian Pat. Nos. 944,535, issued Apr. 2, 1974;959,628, issued Dec. 24, 1974; 959,821, issued Dec. 24, 1974; and963,633, issued Mar. 4, 1975.

The use of hydrophobic treated catalyst particles in other contexts isalso known. See, for example, Stevens Canadian Pat. Nos. 907,292, issuedAug. 15, 1972, and 941,134, issued Feb. 5, 1974. In the former, Stevensdiscloses using a catalyst having a coating directly thereon to promotea process for enriching a fluid with hydrogen isotopes. In the latter, aporous hydrophobic support material having catalyst particles thereon isused in such a heavy water production process.

See also, Butler Canadian Pat. No. 700,933 and Fleck U.S. Pat. No.2,722,504. Butler describes a system for the electrolysis of sodiumchloride brine in which the cathode is porous and supplied with oxygengas to prevent formation or evolution of hydrogen. In one form, thecathode compartment contains a slurry of particulate solids which isagitated by the air stream or by mechanical agitation. The particulatematerial may be graphite and coated with a hydrophobic material such aspolytetrafluoroethylene. The Fleck patent relates to conductingpetro-chemical reactions with a solid catalyst material that has a minoramount of silicone resin on its surface. The vapor phase reactionsdisclosed in Fleck are isomerization, desulfurization, hydrogenation,hydroforming, reforming hydrocracking, destructive hydrogenation and thelike.

All of these patentees apparently use a uniform mixture of treatedcatalysts in terms of the amount of hydrophobic material on the catalystparticles. Thus, several suggest applying a certain weight percentwithin a range, but none disclose any differential application ofhydrophobic material nor is one likely to be achieved absent specializedprocedures to do so. This is an important consideration.

As disclosed in the Smith and Sanders patents, a preferred manner ofusing the wetproofed catalyst particles is to pack them in a fixed bedwithin a tower reactor. The reactants are then flowed eitherconcurrently or countercurrently through the reactor. The flowcharacteristics of the reactants through the reactor depend somewhat onthe amount of hydrophobic material deposited on the catalyst particles.The catalysts treated with higher weight percent amounts of hydrophobicmaterial offer less resistance to fluid flow than those having beentreated with lower weight percent amounts. But, generally the higher theamount of hydrophobic material, the more the catalyst surface isoccluded and the less reactive the catalyst.

In the production of polysulfide, for instance, both the activity of thecatalyst and flow resistance characteristics are important commercialconsiderations in terms of the efficiency of the reactor and the amountof polysulfide produced. Accordingly, the need exists for a wetproofedcatalyst which will decrease the flow resistance characteristics withoutsignificantly decreasing the reactivity -- that is, a wetproofedcatalyst which is more efficient than the uniformly treated catalysts ofthe prior art.

SUMMARY OF THE INVENTION

That need is fulfilled by the wetproofed catalyst particles produced bythe method of the present invention. The polytetrafluoroethylene (PTFE)treated carbon particles prepared by the instant process are uniquelysuited for use in a packed bed column reactor, giving an improvedefficiency when compared to the wetproofed catalyst particles of theprior art. This is believed to be because of the non-uniform coatingachieved with this process so that even when a fixed weight percent ofPTFE is measured out and applied, not all the carbon particles will havethat amount of PTFE on their surface. Some will have a larger amount ofPTFE, some a lesser amount. The result is what appears to be somewhatrandom mixture of PTFE weight percent coatings. This is to be alsodistinguished from a fixed mixture of uniformly coated catalystparticles having different amounts of hydrophobic material on theirsurfaces.

It is achieved by controlling the volume of water in the PTFE emulsionwhich is applied to the carbon particles in an amount less than thatwhich will be picked up by the absorbent carbon particles. It istheorized that only the first portion of the carbon particles coming incontact with the water in the mixing vessel absorb most of the water. Asthey become water saturated the PTFE solids in the aqueous emulsion willbe carried with that water and deposited primarily on these initialwater-absorbing carbon particles. The carbon particles for which thereis not enough water remaining for saturation will have a lesser chanceof contact with the PTFE solids and so will have lesser amounts coatedon their surfaces.

Various degrees of coating are believed to be produced in this manner aswell, although the exact figures are not known. Under microscope somecarbon grains seem to be almost completely encapsulated with PTFE andothers have only small spots of PTFE widely scattered.

The carbon particles used may be either activated or unactivated carbon.Preferred are ones having a particle size of between 2 and 30 mesh andmost preferred are 4 × 10 mesh particles. The weight percentage (on adry solids basis) of PTFE (averaged over all the carbon particles) whichmay be applied in this manner may be varied, but preferred are amountswithin the range 0.6-20 percent. More preferred is 2-10 percent and mostpreferred is 6-8 percent.

The desired amount of PTFE solids is determined and the volume ofemulsion containing that amount weighed out. This is then diluted withwater to give a liquid volume to carbon particle weight ratio of lessthan that required for saturation. In the case of the preferred carbonparticles of the size mentioned this is less than approximately100ml/100 grams. At this liquid level or below there is less liquidpresent than the saturation capacity of the carbon particles.At lessthan approximately 50ml/100 grams or even approximately 25ml/100 gramsthere is even a greater shortage of water present and so even a morenon-uniform mixture of wetproofed carbon particles results. As long asthere is sufficient liquid volume to maintain the PTFE solids insuspension and form flowable liquid, then the non-uniform distributionof the present invention is possible. This level may be as low as0.5ml/100 grams.

The proper volume of aqueous emulsion as determined per the aboveconsiderations is mixed with the carbon particles. This may be simply byadding the liquid to the carbon particle mass and stirring, or sprayingthe liquid onto the particles and tumbling, or combinations thereof, orany other mixing procedure. The actual mixing method is unimportantsince the control of liquid volume introduced and the absorbency of thecarbon will in and of themselves give the non-uniform PTFE distributionon the catalyst particles.

Next, the carbon particles after being mixed with the PTFE emulsion areheated for a sufficient period of time to drive off the water from theaqueous emulsion and to set the PTFE resin as deposited non-uniformlyand non-continuously on the carbon particles. The heat may also driveoff some of the wetting agent present in the PTFE emulsion, but this isnot critical since once dried by water removal, the resin issufficiently set and any residual wetting agent will be washed out inuse.

The result is the mixture of non-uniformly coated carbon particlesdesired. It has been found that such particles when used in a columnreactor for sodium polysulfide production, for example, increase theefficiency in terms of percent of sodium sulfide converted topolysulfide rather than thiosulfate and the rate of polysulfideproduction. In polysulfide pulping an increased conversion topolysulfide is significant commercially. As an illustration, in a 500ton per day hardwood kraft pulp mill using a white liquor sulfidity of25% and 16% active alkali applied on the O.D. wood, a 60% polysulfideoxidation efficiency would give a 0.89% S° on O.D. wood and a 1.07% (11tons/day) increased pulp production. However, a 70% efficiency wouldgive a 1.04% S° O.D. wood and a 1.25% (13 tons/day) increased pulpproduction. While an 80% efficiency gives a 1.19% S° on O.D. wood and a1.43% (15 tons/day) increased pulp production. The wetproofed carbonparticles of the present invention will give a 10% or more improvementin polysulfide oxidation efficiency compared to uniformly coatedcatalyst particles.

Accordingly, it is an object of the present invention to providenon-uniformly PTFE coated carbon particles of improved efficiency.

Another object of the present invention is to provide a method forpreparing such non-uniformly PTFE coated carbon particles.

Other objects and advantages of the invention will be apparent from thefollowing description and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred method of the instant invention is as follows:

a. Fifty pounds (22,680 grams) of carbon such as 4 × 10 BPL activatedcarbon from Pittsburgh Activated Carbon Co. is placed in a Marion Mixer,manufactured by Rapids Machinery Co., Marion, Iowa.

b. PTFE emulsion is introduced through a nozzle. For an approximately 6%by weight of PTFE dry solids at a 25 ml/100 gram liquid amount, thiswould mean that 1500 ml of a 60% solids (at a 1.5 specific gravity) PTFEemulsion such as Teflon Suspension 852-200 from E. I. duPont de Nemours& Co. is diluted with approximately 4200 ml water to give 5700 ml liquidwhich is added to the carbon particles.

c. The combined carbon particles and PTFE emulsion are stirred in theMarion Mixer for approximately 4 minutes.

d. After mixing, the treated particles are dumped in a shallow tray andput into an oven heated at approximately 375° F.

e. The treated particles are heated for around 30 minutes or until dry(i.e., the water from the emulsion is driven off).

f. Finally, the treated particles are removed from the oven and cooled.They are now ready for use.

The resultant particles have a non-uniform, non-continuous PTFE coatingon the carbon. Some show almost but not quite complete PTFEencapsulation, in others very little PTFE appears on the surface, andothers have intermediate amounts. Since at liquid levels of less than100 ml/100 grams there is no excess water (but rather a deficit) beyondthe saturation point of carbon absorption, no liquid is drained off.Therefore, all of the PTFE ends up coated on the surface of the carbonparticles. However, a random non-uniform, non-continuous coatingresults.

Of course, materials and amounts other than the most preferred as givenin the illustration above are usable. Other carbons within the range2-30 mesh may be used; another example is 12 × 30 BPL carbon; another is8 × 30 mesh SGL carbon. As mentioned previously the PTFE percent on adry solids weight basis is preferably between 0.6 and 20 percent. The60% solids duPont material mentioned is also only preferred. Others suchas Fluon GP-1 from ICI United States, Inc., Wheeling, Ill., could alsobe used. Teflon Suspension 852-200 is a dispersion of hydrophobic,negatively charged colloids containing particles of 0.05 to 0.5 micronsdiameter suspended in water. It contains a small amount (around 6%) ofTriton X-100 from Rohm & Haas, Bristol, Pa., as a non-ionic wettingagent.

The emulsion is diluted with water to give the proper amount for mixingwith the carbon particles. Critical is the fact that the amount of wateradded does not result in a liquid addition of over 100 ml/100 grams.More preferred is a liquid amount of less than 50ml/100 grams and mostpreferred is approximately 25ml/100 grams. If the 100ml/100 grams limitis exceeded then there will be generally more than enough water tosaturate all of the carbon particles. As such, unless very specializedmixing procedures are used, an essentially uniform non-continuous PTFEdistribution through the carbon particles will result. The carbonparticles will not be encapsulated by the PTFE, but rather each intheory would have the chosen weight percent (such as 6%) PTFE on itssurface. While this produces a workable wetproofed catalyst, thatmaterial does not have the unique features of the present one.

The mixing time and method are not critical. Variations thereof havefailed to evidence any substantial effect on the wetproofed carbonproduced unless exotic procedures are utilized. Likewise, the oventemperature and heating time are also not critical as long as themelting point of the PTFE is not exceeded. All that is important is thatit be long enough and at a sufficient temperature to drive off themoisture present. Obviously, the lower the temperature used, the longerthe heating time necessary.

EXAMPLE

Using the above method various sets of wetproofed carbon particles wereprepared. In each 4 × 10 BPL activated carbon was used and all that wasvaried was (a) the percent PTFE resin on a dry solids basis and (b) theamount of liquid in ml per 100 grams of carbon particles. This is setforth in Table I below. All other procedures as illustrated above werethe same.

    ______________________________________                                Liquid    Material No.   % PTFE       ml/100g    ______________________________________    1              4.2          75    2              6.5          100    3              6.5          50    4              2.0          50    5              2.0          100    6              7.5          75    7              0.9          75    8              4.2          125    9              4.2          25    10             7.5          25    ______________________________________

Those sets of wetproofed carbon particles were then used in a pilotreactor for oxidizing sodium sulfide liquor to produce sodiumpolysulfide. Synthetic liquors were prepared. The reaction conditionsare set forth in Table II, below:

                  TABLE II    ______________________________________    Bed Depth            11.7 ft.    Liquor Flow Rate     7.5 GPM/Ft..sup.2    Inlet Liquor Temperature                         180° F    Inlet Liquor Sulfide Conc.                         .75M                          Synthetic Liquor    Inlet Liquor Hydroxide Conc.                         2.0M    Air Flow Rates 10,20,30,40,50    and 60 SCFM/Ft..sup.2    Concurrent Down Flow Configuration    ______________________________________

The output was measured for polysulfide production and the results areset forth in Table III as follows:

                                      TABLE III    __________________________________________________________________________    Material #1               % PTFE 4.2  Liquid 75ml/100g.    Air Flow Rates    SCFM/FT.sup.2               10    20    30    40    50    60    Polysulfide Pro-    duced Moles               .076  .161  .218  .246  .272  .226    As glp S.sup.o               2.43  5.15  6.98  7.87  8.70  7.23    Material #2               % PTFE 6.5 Liquid 100 ml/100g.    Air Flow Rates    SCFM/FT..sup.2               10    20    30    40    50    60    Polysulfide Pro-    duced Moles               .054  .102  .085  .077  .064  .057    As gpl S.sup.o               1.73  3.26  2.72  2.46  2.05  1.82    Material#3 % PTFE 6.5 Liquid 50ml/100g.    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Pro-    duced Moles               .080  .164  .241  .292  .327  .296    As gpl S.sup.o               2.56  5.25  7.71  9.34  10.46 9.47    Material #4               % PTFE 2.0 Liquid 50ml/100g.    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Pro-    duced Moles               .069  .166  .246  .290  .316  .302    As glp S.sup.o               2.21  5.31  7.87  9.28  10.11 9.66    Material #5               % PTFE 2.0 Liquid 100ml/100g.    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Pro-    duced Moles               .090  .164  .224  .256  .270  .248    As glp S.sup.o               2.88  5.25  7.17  8.19  8.64  7.94    Material #6               % PTFE 7.5 Liquid 75ml/100g    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Pro-    duced Moles               .086  .167  .202  .220  .201  .208    As gpl S.sup.o               2.75  5.34  6.46  7.04  6.43  6.66    Material #7               % PTFE 0.9 Liquid 75ml/100g    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Produced    Moles      .100  .198  .270  .327  .322  .241    As gpl S.sup.o               3.20  6.34  8.64  10.46 10.30 7.71    Material #8               % PTFE 4.2 Liquid 125ml/100g    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Produced    Moles      .074  .164  .194  .194  .160  .160    As gpl S.sup.o               2.37  5.25  6.21  6.21  5.12  5.12    Material #9               % PTFE 4.2 Liquid 25ml/100g    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Produced    Moles      .104  .203  .278  .348  .352  .299    As gpl S.sup.o               3.33  6.50  8.90  11.14 11.26 9.57    Material #10               % PTFE 7.5 Liquid 25ml/100g    Air Flow Rates    SCFM/Ft..sup.2               10    20    30    40    50    60    Polysulfide Produced    Moles      .118  .235  .322  .369  .364  .334    As gpl S.sup.o               3.77  7.52  10.30 11.81 11.65 10.69    __________________________________________________________________________

Runs with materials Nos. 2 (liquid = 100ml/100g) and 8 (liquid =125ml/100g) illustrate what happens when a nearly uniform PTFEdeposition (because of the higher amounts of liquid used) is achieved.The amounts of polysulfide produced in these runs is in almost allinstances below that produced in the other runs. Likewise the peakpolysulfide production (at the individual optimum air flow) in eachinstance is lower than with the other runs. Run 5 also used 100ml/100gbut at the lower PTFE amount the effect was not as great as in runs 2and 8. All of the remaining runs were cases using well less than100ml/100g. liquid and all performed with improved efficiency(particularly at the optimum air flow for each).

While the method and article herein described constitutes a preferredembodiment of the invention, it is to be understood that the method andarticle is not limited to this precise method and article, and thatchanges may be made therein without departing from the scope of theinvention which is defined in the appended claims.

What is claimed is:
 1. A method for preparing wetproofed carbonparticles comprising:a. mixing a measured amount of carbon particleshaving a size between 2 and 30 mesh with 0.6 - 20 percent by weightpolytetrafluoroethylene resin,said polytetrafluoroethylene resin beingin an aqueous emulsion, and said aqueous emulsion being added to saidcarbon particles in amounts of less than approximately 100ml/100 grams,and b. heating for a sufficient period of time to drive off the waterfrom said aqueous emulsion and thereby setting saidpolytetrafluoroethylene resin as deposited non-uniformly on said carbonparticles to yield a mixture of polytetrafluoroethylene coated carbonparticles, some having a higher weight percent polytetrafluoroethyleneresin thereon and others having a lower weight percentpolytetrafluoroethylene resin thereon.
 2. A method as in claim 1 wherein2 - 10 percent by weight of polytetrafluoroethylene resin is used.
 3. Amethod as in claim 2 wherein said aqueous emulsion is added to saidcarbon particles in amounts of less than approximately 50ml/100 grams.4. A method as in claim 3 wherein 6 - 8 percent by weight ofpolytetrafluoroethylene resin is used and said aqueous emulsion is addedto said carbon particles in an amount of approximately 25ml/100 grams.5. A method as in claim 4 wherein said carbon particles areapproximately 4 × 10 mesh in size.
 6. A product produced by the methodof claim 1.